The present invention relates to a radio communication system used for mobile communications or the like, and more specifically to an adaptive array antenna appropriately used for multi-carrier radio communications in which wide band signals are transmitted via a plurality of carrier waves.
Conventionally, the adaptive array antenna is known as an antenna for receiving radio waves while suppressing jamming signals such as interference waves in mobile tool communications or the like. The antenna of this type, as shown in FIG. 1, for example, direct its beam to the direction of a desired wave, and automatically forms such a pattern of null (point zero) in the direction of a jamming wave, from a received signal.
An adaptive array antenna basically consists of a plurality of antenna elements. Signals received by these antenna elements are subjected to an appropriate weighting, and the weighted signals are synthesized to form a synthesized pattern as can be seen in FIG. 1. As examples of the method of determining the amount of weighting, the LMS (least means square) method in which the weighting is determined such as to minimize the root error component of an actually received signal with reference to a known reference signal, the MSN (maximum signal to noise ratio) method in which the SN ratio is maximized with a known direction of a desired wave, the PI (power inversion) method in which a strong jamming wave signal is suppressed by the minimization of received power in, for example, reception of a frequency diffusion modulation signal, and the CMA (constant modulus algorithm) method in which an interference wave is suppressed by utilizing the constant envelope property of an amplitude component of a frequency-modulated or a phase-modulated signal, are proposed.
As an example of the prior art technique, the structure of an LMS-type adaptive array antenna is illustrated in FIG. 2. As shown in this figure, an weighting device 12 is connected to each of a plurality of antenna elements 11, and signals received by these antenna elements respectively are individually weighted. These weighted signals are added up by an adder 13, and then outputted. The output obtained by the addition (ADD output) is supplied to a receiver, and at the same time, to a reference signal generating circuit 14, where a reference signal having a strong correlation with a desired signal, and having a weak correlation with a jamming wave, is generated out of the ADD output.
In the case where the reference signal is known, the step of extracting the reference signal from the ADD output, can be omitted. Subsequently, the difference between the ADD signal and the reference signal is output by a subtracter 15 as an error signal, and then input to a control circuit 16. The control circuit 16 determines the weight for each and individual antenna element on the basis of the error signal, and controls each weighting device 12.
Thus, a feedback operation is carried out by establishing the above structure, and consequently, an synthesized antenna pattern obtained with the weighting assigned to each antenna, forms a null in the direction of the jamming wave. As a result, the antenna pattern has a high gain in the direction of the desired wave.
Incidentally, the multi-media is presently brought into focus in general, and similarly, in the field of radio communications, the multi-media communications are brought into focus. In the hear future, a greater capacity of data not only of audio, but also of image or even other data, will be transmitted via radio. Therefore, the reference band width of a wave per one piece of data (channel) is considered to be widened in the future. In order to transmit a wide-band signal, it is essential to enhance the wide-band characteristics of a hardware device such as a receiver; however such a requirement is not easy to achieve.
Under these circumstances, as a solution to the above problem, a multi-carrier transmission mode for transmitting a piece of data by dividing it into several bands and putting them on carriers having different frequencies, has been proposed.
However, regarding the communication system of the multi-carrier transmission mode as described above, in order to constitute an adaptive array antenna for receiving a desired wave efficiently by suppressing an interference wave, it is necessary to provide weighting devices in the same number as that of carrier frequencies, for the antenna elements, as well as circuits for controlling the antenna elements in the same number. As a result, the structure of the antenna becomes very large and complicated.
The object of the present invention is to provide an adaptive array antenna and a multi-carrier radio communication system, which has a simple structure and is capable of being controlled easily in a short period of time and of effectively suppressing unnecessary waves.